Monday, February 28, 2014

Microtubules Introduction

1Microtubules¶

1.1Where Do We Find it and what do we use it for?¶

• Provide structure to cell
• Provide long range transport
• It is hollow and surrounded by 13 protofilaments
• Used in the cell cycle to build up the mitotic spindles.
• Found in neurones
• Found in cilia (assemblies of microtubules)
• Microtubules are also used as ‘roadways’ for intracellular transport.
• Cells have ability to mark different microtubules as prefential which allows it to conduct efficient vesicular transporation.
• The microtubules also form the mitotic spindle which allows the cell to split during cell division.
• Flagella are also cilia and thus are made of microtubules.
• Microtubules also form the backbone of the neurone.

1.2How is it Organized?¶

• Microtubules in animal cells are organized by the centrosome.
• Microtubules are continuously build and broken down and rebuilt.
• Microtubules in neurones are tightly packed.

1.3How is it Built?¶

• Microtubules are built from tubulin heterodimers of alpha tubulin and beta tubulin. Tubulin is a GTPase.
• Both have a nucleotide in them (GTP or GDP). The GTP is put in the molecule in the binding process but in the alpha subunit it will never be hydrolyzed or exchanged. In the beta subunit the GTP will be hydrolyzed. Thus when we refer to the hydrolysis of GTP in the microtubule cycle we are referring to the beta subunit.
• Rate constants of the microtubule polymerization process are not as clear as with actin. However, we do know that they have a (+) end and a (-) end.
• The end of the protofilament is stabilized by GTP-cap.
• With the loss of the GTP-cap the microtubule will switch to result in rapid shrinking
• GDP-tubulin is kinked (not straight) and thus it will flair out and bend out.
• If the GTP-cap is lost, there is a switch from growth to rapid shrinking.

2Dynamic Instability¶

• This allows cells to quickly restructure their microtubules.
• It was first deduced and then observed directly.